Froth flotation is commonly used in the mining industry to recover mineral values from aqueous ore slurries. A wide variety of suitable frothing agents (“frothers”) have been identified, although the best frother for a particular application is usually selected through experience or by trial and error. Alkyl or aryl ethers of propylene glycol and polypropylene glycols have long been generally known as effective frothers for copper recovery (see, e.g., U.S. Pat. Nos. 2,611,485, 2,695,101, and 3,595,390).
The South African mining industry uses tripropylene glycol methyl ether (TPM) as a component of frothers for recovering platinum and other precious metals. While the product performs well, it is produced commercially as a by-product of the normal process for making propylene glycol methyl ether (PM) from methanol and propylene oxide. Usually, TPM is recovered after a labor-intensive series of steps that includes base-catalyzed alkoxylation, distillations to recover PM and dipropylene glycol methyl ether (DPM), water extraction of the distillation residue (known as “DPM column bottoms”) to remove the basic catalyst, and multiple distillations to recover purified DPM and TPM. Consequently, TPM is expensive and in relatively short supply. Unfortunately, demand for TPM is still not sufficient to justify its “on purpose” manufacture.
The mining industry, particularly the platinum mining industry, would benefit from the availability of inexpensive alternatives to TPM that provide acceptable performance as frothers. Not all glycol ether compositions are suitable for use in platinum recovery. For example, our own evaluation of ethoxylated PM demonstrated unacceptable frothing performance.
Recently, we described (U.S. Pat. No. 7,482,495) a way to make glycol ether compositions that are useful for metal recovery. Reaction of PM with from 1.5 to 3 equivalents of propylene oxide (PO) provides an initial alkoxylation mixture that is generally unsuitable for use as a frother. Distillation of this material, however, to remove some of the DPM affords a composition comprising at least 30 wt. % of TPM and less than 20 wt. % of DPM, and this distilled product performs well in tests designed to predict performance in frother applications. Moreover, similar results can be achieved by reacting DPM with from 0.5 to 1.5 equivalents of PO, followed by distillation, to make an analogous product.
Despite the success of the compositions of the '495 patent and their advantages over highly purified materials such as TPM, there is room for improvement. The fractional distillation used to reduce the DPM level is cumbersome and requires diligent sampling and analysis to ensure that the product will meet targeted specifications. A single batch of off-spec material can trigger substantial process tweaking and product blending to generate a finished product that meets specifications. Moreover, distilling and recycling DPM is energy-intensive and unproductive; the need to recycle DPM reduces batch yields by 20% or more. Preferably, the process would avoid the need for a distillation step.
Thus, a valuable process would overcome the need to: (1) start with relatively pure PM or DPM (each of which is obtained after multiple distillations); (2) add an alkoxylation catalyst; or (3) perform multiple distillations to purify the alkoxylated product.
Finally, better frothers are always desirable. The best frothers generate a stable froth when an ore-containing liquid mixture is aerated in the presence of a small proportion of frother. Metal values are recovered by separating the froth from the bulk of the liquid mixture. A valuable frother provides a froth of limited stability such that removal of aeration results in rapid collapse of the froth and permits easy isolation of the metal components. An ideal process would provide frothers that meet or exceed the performance of commercial frothers, including TPM.